Signal transformation involving alpha-adrenoceptors

Based on different affinities of various agonists and antagonists, alpha-adrenoceptors are subdivided into two subtypes, alpha 1 and alpha 2. Stimulation of alpha 1-adrenoceptors induces several changes, possibly acting as intracellular signals triggering the final cellular response. Among these changes, the increase in the cytosolic calcium ion concentration appears to be the most important signal. Increased membrane phosphatidylinositol turnover and increased cyclic guanosine 3', 5'-monophosphate (cGMP) levels under alpha 1-adrenoceptor stimulation may play a role as signals primary and secondary to the increased calcium influx, respectively. The primary intracellular signal induced by stimulation of alpha 2-adrenoceptors appears to be a fall in cellular cyclic AMP levels as a consequence of inhibition of membrane-bound adenylate cyclase. The molecular mechanisms underlying the alpha 2-adrenoceptor-mediated inhibition of this enzyme are only poorly understood, and apparently involve a guanine nucleotide-binding coupling component, which may be in part different from that involved in beta-adrenoceptor-mediated stimulation of the adenylate cyclase.